Process of making low-carbon chromium alloys



Patented Feb. 7, 1933 UNITED STATES teem? PATENT oFFlcs FEEDERIGK M. IBECKET, OF NEW YORK, N. Y., ASSIGNOR T0 ELECTRO METALLURGICAL GOMZPANY, A. COR POBATION OF WEST VIRGINIA PROCESS OF MAKING LOW-CARBUN CHROMIUM ALLOYS No Drawing.

This invention relates to the art of making low-carbon chromium alloys typified by rust-; less iron. lhe object of the invention is to provide a novel and economical process to preparing such alloys.

Rustless iron for the purposes of this invention may be defined as an alloy consisting essentially of iron and chromium, with upward of about 9% of chromium and less than 0.2% of carbon. Nickel may also be present in varying proportions replacing the iron. A preferred composition contains about 12%1 l% of chromium with 0.12% or less of carbon. Another excellent composition contains around 16%18% of chromium, with 7%8% nickel and carbon around 0.12% or less. 7

Such alloys have heretofore been made by various processes, such as the introduction of low-carbon ferrochrome or chromium metal into a bath of low-carbon iron; or by subjecting an iromchromium alloy containing more than the desired proportion of carbon to an oxidizing treatment to eliminate a portion of the carbon, and thereafterde oxidizing the resulting low-carbon product, as disclosed in United States Patent 1,865,091 to (Element.

In the process disclosed in the Clement pat- V ent, the final step of deoxidizing the metal is necessary because of the oxygen which the bath takes up in the decarburizing stage.- This oxygen content and the necessity for its removal are discussed in the patent on page 3, beginning at line 100.

In an extensive experience with the Glennent process I have confirmed the patentee as to the ability of his process to produce rustless alloys of good quality, and have in fact demonstrated that the process is in all respects a meritorious one. I have nevertheless developed certain improvementsin the process, and these improvements form the subject of the present application.

In the Clemenat process, deo xidation of the metal is'eifected by means of a so-called carbide slag, prepared by removing the slag used in the preceding stage, covering the surface of the metal with lime and melting down the latter at a very high temperature,

Serial No. $35,009.

tion can advantageously be carried out by means other than those described in the patent.

I The use of a carbide slag as described in the patent necessitates very high temperatures andv thorough agitation of the metal with the slag. These conditions favor the oxidation of chromium and the losses of the latter are apt to be rather high. In addi- 1 tion great care must be exercised in forming the slag to avoid recarburizing the metal. Y

I prefer to deoxidize with a reducing agent which is non-carbonaceous, whereby all risk of recarburizing the bath is avoided. In some cases the object is to diminish the carbon content of the metal to 0.05% or even less, and in View of the avidity with which chromium-alloys take up carbon it is necessary to use'ever-y precaution to avoid introduction of carbon in the later stages of the process.

lhe deoxidizing agents which I employ act more rapidly than the carbonaceous materials described by Clement, and because of this more rapid deoxidation the time is diminished during which the heat is subjected to the carbon-bearing atmosphere of the furnace, this being an advantage of some importance where metal of a very low carbon content is the objective. I prefer to use silicon or its alloys as deoxidizers, for example ferrosilicon, or silicon alloys containing one or more of the elements chromium, manganese, magnesium, calcium, zirconium, aluminum or titanium, with or without considerable quantities of iron. The invention is not restricted to the use of silicon alloys, however, and other deoxidizers may be used. A plurality of deoxidizers sometimes gives the best down together and 580 parts of high-carbon ferrochrome containing Cr 69.2%, C 4.79%, and Si 1.80% was then added, meanwhile keeping the bath fluid. This was followed by 100'parts of chrome ore added in three portions, one hour intervening between additions. About an hour after the last addition of ore the carbon had fallen to 0.06% and the first slag was poured off.

A new slag was then formed, the ingredients and their quantities being the same as before. When this slag had become fluid, 60 parts of ferrosilicon was added infour lots at intervals of 5 minutes. Shortly after the last addition 17 parts of low-carbon fer-' romanganese (Mn 82.5%, Si 1.03%, C 0.13%) was added and the heat tapped and poured into ingot molds. Only '85 minutes ela sed between the removal of the first slag an the tapping of the metal.

The finished metal contained Cr 12.13%, C 0.06%, Si 0.18% and Mn 0.35%. The metal poured well and all the ingots had good shrinkage.

The substitution of a metallic deoxidizin-g agent for a carbon-containing slag, as described above, permits a further modification of the Clement process under some conditions. This modification consists in the elimination of one of the slags, and it can be followed when the raw materials used in making the rustless iron are very low in phosphorus and sulphur. In the one-slag process the deoxidizer is introduced into the furnace at the end of the decarburization, without previously tapping the slag. Under these conditions there is a tendency for phosphorus and sulphur to pass from the slag to the metal, and

the content of these elements in the slag at the end of the decarburizing stage must therefore be low. The one-slag process is especially applicable Where an extremely low carbon content is notrequired, but it may be used even in this case under proper conditions. When the single-slag modification can be used a considerably increased recovery of chromium can be achieved because chromium is returned to the metal from the decarburizing slag.

The following example illustrates the mod ification last described: I

Example I I .Lime (200 parts) and quartzite (100 parts) were melted down in a basic lined electric furnace, and 970 parts of highgrade low carbon steel scrap and 280 parts of high-carbon ferrochrome were then added.

The ferrochrome analyzed Cr 7 0.62%; C 6.51%; Si 1.15%. Forty parts of chrome ore was then added to the slag and the bath was maintained at high temperature until the carbon was slightly less than 0.10%. Fortyfive parts of 12 meshferrosilicon was then scattered orf the slag in successive small portions, 5 pounds of lime being also introduced for each pound of Si in the ferrosilicon thus added, thus maintaining the basicity of the slag. When the metal was sufiiciently deoxidized, a small amount of manganese was introduced by the addition of 10 parts .of low-carbon ferromanganese, and'10 minutes later the heat was tapped. A small quantity of silicon-zirconium was ,added to the ladle in accordance with established steel-making practice. The alloy analyzed Cr 13.17%; C 0.10%; Si 0.38%; Mn 0.54%. It will be noted that in the foregoing example, the number of pounds of silicon introduced as ferrosilicon during the process is greater than the "number of pounds of chrome ore charged to the furnace. Therefore the silicon added is largely in excess of that required to reduce theoxides of chromium and iron contained in the ore. This results in a reduction of a high proportion of the chromium in the slag, greatly increasing the recovery of chromium.

The procedure given in the Example II may be advantageously modified by addingc a very energetic. eoxidizer such as aluminum to the.metal in the furnace after the action of the first deoxidizer (ferrosilicon) is substantially complete. The quantities of silica used in the specific examples for making up the slags tend to facilitate the manipulation of the furnace, but slags of a more basic character expedite deoxidation of the metal and are preferable on that ground.

The term high-carbon ferrochrome as used in the specification and claims refers to a product characterized by a relatively high ratio of carbon content to chromium content, and made by reducing a chromium-oxide-containing material with carbon under-conditions permitting carburization of the reducedmetal. Where the appended claims refer to combining high-carbon ferrochrome with iron it will, of course, be understood that the materials may be combined either directly or indirectly, and in any one of a'number of different ways, some of .which are enumerated in the Clement Patent Number 1,365,091.

I claim:

1. The process of making iron-chromium alloys low in carbon which comprises combining high-carbon ferrochrome with low carbon iron to form a melt having a chromium content at least as high as that desired in the finished alloy; said melt containing an excess of carbon and being covered by a slag; adding chromite to said slag, maintaining the melt at a high temperature under the slag to .rochrome with iron to form a melt in which the ratio of the carbon content to the chromium content is materially higher than in rustless iron, providing on said melt a covering slag containing an oxide of a metal of the group consisting of iron and chromium, maintaining the melt at a high temperature thereby oxidizing the excess carbon and incidentally a substantial part of the chromium in the melt, the chromium oxide thus formed passing into the slag, and then, without removing the slag, subjecting the slag to the action of a non-carbonaceous reducing agent whereby to reduce a substantial proportion of the chromium oxide content thereof.

3. The process of making rustless iron which comprises combining high-carbon "ferrocrome with iron to form a melt in which the ratio of the carbon content to the chromium content is materially higher than in rustless iron, providing on said melt an oxidizing covering slag containing an oxide of a metal of the group consisting of iron and chromium, maintaining the melt at a high temperature thereby oxidizing the excess carbon and incidentally a substantial part of the chromium in the melt, the chromium oxide thus formed passing into the slag, and then, without removing the slag, subjecting the slag to the action of silicon whereby to reduce a substantial proportion of the chromium oxide content thereof.

4. The process of making rustless iron which comprises combining high-carbon terrochrome with low-carbon iron to form a melt in which the ratio of the carbon content to the chromium contentis materially higher than in rustless iron, providing on said melt a basic lime silica slag containing chromium oxide, maintaining the melt under said slag at a high temperature thereby oxidizing the excess carbon and incidentally a substantial part of the chromium in the melt, the chromium oxide thus formed passing into the slag, and then, without removing the slag, and while keeping it in a basic condition, subj ecting the slag to the action of silicon to reduce a substantial proportion of the chromium oxide content thereof.

5. The process of making rustless iron which comprises combining high-carbon ferrochrome with low-carbon iron to form a melt in which, the ratio of the carbon content to the chromium content is materially higher than in rustless iron, and in which the chromium content is at least as high as that desired in the finished alloy, providing on said melt a slag containing chromium oxide, maintaining the melt under said slag at a high temperature thereby oxidizing the excess carbon and incidentally a substantial part of the chromium in the melt, the chromium oxide thus formed passing into the slag, and then, without removing the slag, and while keeping it in a basic condition, subjecting the slag to the action of a noncarbonaceous reducing agent whereby to reduce a substantial proportion of the chromium oxide content thereof.

6. lhe process of making rustless iron which comprises combining high-carbon terrochrome with iron to form a melt in which the ratio of the carbon content to the chromium content is materially higher than in rustless iron, said melt being covered by a slag, adding chromite to said slag, maintaining the melt at a high temperature thereby oxidizing the excess carbon and incidentally a substantial part of the chromium in the melt, the chromium oxide thus formed passin g into the slag, and then, without removing the slag, subjecting the slag to the action of a noncarbonaceous reducing agent whereby to reduce a substantial proportion of the chromium oxide content thereof,

7. The process of making rustless iron which comprises combining high-carbon terrochrome with low-carbon iron to form a melt in which the ratio of the carbon content to the chromium content is materially higher than in rustless iron, and in which the chromium content is at least as high as that desired in the finished alloy, said melt being covered by a basic lime silica slag, adding chromiteto said slag, maintaining the melt under said slag at a high temperature thereby oxidizing the excess carbon and incidentally a substantial part of the chromium in the melt, the chromium oxide thus formed passing into the slag, and then, without removing the slag, and while keeping it in a basic condition, subjecting the slag to the action of silicon to reduce a substantial proportion of the chromium oxide content thereof.

in testimony whereof, I afix my signature.-

FREDERICK M. BECKET.

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